1. Field of the Invention
The present invention relates to a phase difference measuring method and phase difference measuring device for finding the defects of phase shift masks used in the fabrication of LSI (Large-scale Integrated Circuits).
2. Description of the Related Art
Fabrication of LSI includes a process for forming a desired LSI pattern on a semiconductor wafer using an optical aligner generally referred to as an "optical stepper." This optical alignment process is used in transferring a pattern on a mask to a semiconductor substrate using photolithography. In some cases, a phase shift mask provided with a phase shift portion that transmits while shifting the phase of light 180.degree. is used as an exposure mask for forming a high-density pattern. For improved resolution and high-density pattern formation in such cases, it is essential that the phase difference of light that has passed through the phase shift portion be precisely 180.degree. with respect to the phase of light that has passed through other portions, i.e., the amount of phase shift must be accurate. In this interest, the phase shift mask is examined by directly measuring the actual amount of phase shift of the phase shift mask. According to one method of measurement, light that has passed through the phase shift portion and light that has proceeded along a separate optical path are brought into mutual interference, and the amount of phase shift is calculated based on the brightness or the amount of shift in period of the interference pattern (interference fringes).
The two methods described hereinbelow both involve such production of interference for the purpose of measuring the amount of phase shift. The first method employs a Mach-Zehnder interferometer. As an example, according to a method disclosed in Japanese Patent Laid-open No. 159978/95, light from a light source (mercury lamp) is split by filter, and then light of the wavelength used for exposure is extracted and incidents on a phase shift mask. A sharing member is adjusted so that the light that is passed through a phase shift portion and directed to the Mach-Zehnder interferometer is caused to interfere with light that is passed through a portion other than the phase shift portion and is directed to the Mach-Zehnder interferometer.
The other method does not employ a Mach-Zehnder interferometer, but rather, applies a Nomarski method. In the method disclosed in Japanese Patent Laid-open No. 94444/96, for example, light of a wavelength used for exposure is separated into two portions of radiant flux of differing polarization directions by such as a prism made of birefringence, directed into a phase shift portion and another portion, and transmitted. The polarized light portion of the same direction of both transmitted light beams are then extracted and caused to mutually interfere.
However, in the former method (the method using the Mach-Zehnder interferometer), splitting the light from the light source (mercury lamp) by means of a filter brings about a decrease in the amount of light that can be used, with the result that the alignment of light axes within the Mach-Zehnder interferometer is extremely difficult.
In the latter method, measurement of ultraviolet light that is actually used in exposure that is carried out for a long time may cause deterioration of the birefringent crystal and declining the reliability of the measurement of the amount of phase shift. This method is therefore not practical.